Mobile hydroacoustic upward-looking transducers were used to search invertebrate in upper layers of artificial lake in the Central Europe. Acoustic scattering mainly from Chaoborus larvae and juvenile fish was studied using two split-beam transducer 120 and 38 kHz. Ichthyoplankton trawl was used to identify natural composition of both groups at two different depths 0-2 and 3-5 m. In the targets strength range corresponding to invertebrate apparent peak was found at TS -66 and -64 dB (120 kHz). This range corresponds to average size of Chaoborus mainly caught by ichthyoplankton trawl. The correlation between volume density of Chaoborus established from direct sampling and acoustic recording was high (r<0.73), nevertheless, the correlation was significantly different from 1:1 line.

Information about fish distribution and abundance in the upper layer is often fundamental. However, this information is extremely hard to obtain using hydroacoustic methods. Mobile hydroacoustic upward-looking transducers provide a unique option to survey the upper layers of lakes. We developed a rigid frame system pushing the upward looking transducers of the scientific echo sounder (38 and 120 kHz) in front of the research vessel. The efficiency of the new acoustic approach for monitoring juvenile fish at night was investigated by comparing the results with a quantitative fry trawl in the Římov Reservoir in the Czech Republic. The experimental setup enabled comparisons for the 0-3 m and 3-6 m depth layers, which are utilized by almost all juvenile fish in the summer. No statistically significant differences in the estimated abundance of juveniles were found between the two sampling methods, and the estimated length distribution was similar for both methods.

RS08 – Presentation

Succession dynamics of biofilm and macroinvertebrates following flooding and artificial clear water releases in a Mediterranean-climate river (Durance, France)

The Durance River (southern France) is a highly regulated, gravel-bed river with a naturally high fine sediment load. Flow regulation (8 dams, 16 hydropower plants along 218 km below Serre-Ponçon Dam) has contributed to clogging. Annual clear water releases are used to reduce superficial clogging and are associated with long-term monitoring. The succession dynamics of benthic invertebrates and biofilm in a reach subject to both occasional natural floods and clear-water releases were characterized (natural MAF = 122 m3/s). Riffles were sampled regularly during 4-months following a natural flood (1200 m3/s) and before/after a clear-water release (70 m3/s) and compared to benthic clogging conditions. Clogging remained low (< 25%) 4-months post-flood, but was not changed (50%) following the release, which was therefore of limited efficacy. Biofilm communities included cold-water, rheophilic diatoms (e.g. Hydrorus foetidus) immediately post-flood, followed by the arrival of cyanobacteria. Filamentous algae (Cladophora) dominated in all campaigns post-release. Invertebrate communities were slow to recover post-flood, demonstrating a distinct recolonisation process by species associated with unclogged conditions (e.g. Oligonuriella rhenana). The clear water release did not disturb succession dynamics in 2016 compared to a reference station. The influence of biofilm development on invertebrate composition was also investigated.

SS06 – Poster

Benthic methane fluxes through the sediment-water interface of a lowland river

There is a paucity of data concerning sources of methane (CH4) to the fluvial freshwater ecosystems, despite of their significant contribution to global CH4 flux. River sediments are one of the important sources of CH4 to surface water, however combination of the multiple factors results to high spatiotemporal heterogeneity of CH4 fluxes and their quantification and extrapolation is rather complicated. In this study the investigation of relationships between surface sediments and CH4 supply to the surface water has been conducted for the various river habitats in order to compare the sediment samples with significantly different abiotic and biotic parameters. Three main issues have been measured - environmental parameters of sediments; potential capacity of sediments to produce or consume CH4 and the directly measured CH4 diffusion through the sediment-water interface using benthic chamber method. The CH4 diffusive fluxes to the surface water showed high spatial variability and ranged from 0 to 27.3 mmol CH4 m-2d-1. In general we observed three main model situations of sediment-water interaction. High CH4 fluxes were observed, when the CH4 production potential of upper sediment layer was high, and the CH4 concentration was increased in both upper (0-10 cm) and lower (20-30 cm) hyporheic interstitial water. On the other hand, CH4 diffusive fluxes were significantly lower or zero when either the only upper or the only lower sediment layers were CH4 productive and interstitial water was enriched with CH4. The CH4 oxidation in sediments probably prevents CH4 release to surface water in the last two mentioned cases.

SS10 – Poster

Response of phytoplankton and zooplankton communities to extreme temperature and nutrient stressors: a mesocosm experiment

Phytoplankton and zooplankton are short-lived organisms responding fast and directly to environmental fluctuations which make them useful indicators of deterioration in lake ecosystems to stressors. Due to climate change, extreme events including heat waves will be more common in the future and such changes also augment eutrophication, a problem lakes are already facing today. To understand the effects of temperature and nutrient stressors on phytoplankton taxonomic groups and size diversity we used a mesocosm experiment in Silkeborg, Denmark, a system which has been already continuously operated for 11 years. There are 24 mesocosms (1.9 m in diameter, 1.5 m in total depth, imitating a shallow lake) simulating two nutrient levels (unenriched and enriched with additional nitrogen and phosphorus) and three different temperature scenarios (ambient, IPCC A2 scenario and A2+%50) with 4 replicates. Heat wave was imitated for 1 month by rising the temperature 5°C (from 1st July till 1st August 2014). During this period samplings were done twice a week, and later less frequently. Phytoplankton and zooplankton samples were identified, biovolumes and size diversity were calculated. Using also physico-chemical variables from the mesocosms, the effects of the extreme heat wave on the phytoplankton and communities and structure at contrasting nutrient and temperature levels will be discussed.

RS16 – Presentation

Early investigations into the relationship of the parasitic stage of Margaritifera margaritifera and its fish host in culture.

The life cycle of the critically endangered freshwater pearl mussel (Margaritifera margaritifera) involves a parasitic stage in the gills of suitable salmonid fish. During this period the larvae (or glochidia) increase by more than 4 times their initial size before dropping off into gravel beds.
Over the last two years, in our facility an unusually early larval excystment has been observed. This exposes the newly excysted juveniles to early winter temperatures, potentially reducing their survival rate.
Previous studies have demonstrated how glochidial growth and therefore drop off is stimulated by constant warmer temperatures, as well as reporting a significant variation in timing and length of the excystment periods. Further studies show that differences in numbers and growth rate of attached glochidia indicate strong fish host specificity at species and strain levels.
Our recent observations cannot be simply explained by degree days. Before any significance of temperature can be attributed the suitability of different individuals of the same host population has been investigated. All hosts from the same stock were encysted under the same conditions. Encysted fish were split into length categories, the total number of glochidia in each fish was counted and 30 glochidia measurements were taken from each fish systematically. Fish weight and length were noted for each individual.
The results presented here are a step towards an improved understanding of this host-parasite relationship. In our facility, fish condition is not negatively affecting glochidial development, essential for successful captive breeding and reintroduction

RS07 – Presentation

Relating chemical pollution to biological effects in the field

The contamination of freshwater systems with thousands of industrial, agricultural and household chemicals is a major public concern, but long-term effects on aquatic flora and fauna are largely unknown. It is hypothesized that micropollutants play an important role for the ecological deterioration of streams and failure to achieve good ecological status as demanded by the EU water framework directive.
To explore the relationship between chemical pollution and ecological effects in the field, we compiled a large data set from macroinvertebrate and chemical monitoring programs in Germany. Different macroinvertebrate based indices (ASPT, saprobic index, % EPT, BMWP score, SPEAR %, MMI) indicated ecological status. Indicator substances (i.e. caffeine and carbamazepine) and toxic units with regard to pesticides (TUpesticides) characterized chemical pollution. Using several single and multivariate statistical approaches as well as Threshold Indicator Taxa ANalysis (TITAN) the relative importance of micropollutants compared to traditional physicochemical water quality parameters was evaluated.
Results suggest that traditional water quality parameters such as oxygen depletion and salinity are still major determinants of the ecological quality of streams. However, a clear impact of wastewater and wastewater associated compounds was also apparent, suggesting the need for continued efforts to reduce chemical loads in streams.

Many methodological approaches have been developed to understand and predict how ecosystems function, especially under climate- and human-induced pressures. These approaches range from theoretical to data-driven models, and from field observations to well-controlled laboratory experiments. However, large pelagic enclosure experiments that facilitate statistically sound experimental designs while capturing much of the complexity of aquatic ecosystems are strongly underrepresented. These features have potential to provide mechanistic insights into ecosystem processes that might not be achieved through other approaches such as long-term data acquisition or small-scale experiments with limited species pools. Although experiments on ecosystem-scale levels are powerful, designing and conducting them requires particular care and expertise to obtain meaningful results. Challenges to meet include controlling periphyton growth on enclosure walls and maintenance of natural physical conditions and higher trophic levels over extended periods. These potential problems diminish with increasing size of the experimental units, however, other challenges such as ensuring similar initial conditions and control of fish tend to increase. Moreover, pelagic enclosures, even of large facilities, can reflect natural conditions only to some extent. Knowledge of the specific limitations is hence critical when interpreting and extrapolating the experimental data. Repeated coordinated experiments in multiple facilities could mitigate these limitations and place the results in a broad spatio-temporal context, especially when backed by long-term observational data and a sound theoretical and modeling framework. Further, to gain global insights into aquatic ecosystem functioning it is necessary to build collaborations beyond classical borders, by connecting experimental science in rivers, lakes, estuaries and oceans.

Invasive fishes have been introduced for centuries in alpine Pyrenean lakes. By the year 2000, more than half of the 500 lakes from the Catalan Pyrenees were found to be invaded with major consequences for their biodiversity and functioning. Fish predation reduces amphibian juvenile abundance and may control the densities and sizes of macroinvertebrates and zooplankton. Such changes may also cascade down and threaten macrophyte populations but their potential impacts have not yet been reported. In this work we compare the benthic environment (i.e. sediment deposition, sediment respiration and organic content) and meadow structure (epiphyte load and macrophyte biomass allocation) in five shallow Pyrenean lakes vegetated with Isoetes lacustris. Two lakes were used as a reference without fishes, one with trout, and two with dense populations of Phoxinus sp. We encountered much larger (ANOVA p < 0.001) sediment deposition, sediment respiration and epiphyte load in lakes with Phoxinus sp. as expected from a eutrophication effect. Epiphyte composition from these lakes differed by the presence of low light tolerant cyanobacteria. In these systems I. lacustris showed significant changes in biomass allocation compared to control and trout lakes. These results show that the presence of dense Phoxinus sp. populations leads to a phase change from clear oligotrophic waters towards turbid conditions affecting the quality of water and sediment and threatening I. lacustris populations. Our results indicate that the impact is substantial and might result in a regression of the I. lacustris in these lakes.

RS17 – Presentation

The Effects of Run-of-River Hydroelectric Power Schemes on Fish and Invertebrate Community Composition in Temperate Streams and Rivers

Run-of-river (ROR) hydroelectric power schemes are often presumed to be less environmentally-damaging than large-scale storage schemes. However, there are currently only a limited number of peer-reviewed studies on their physical and ecological impact. This presentation will summarise the findings from a policy secondment, funded by the UK’s Natural Environment Research Council and the Environment Agency of England, which investigated the impacts of ROR hydroelectric power schemes on fish and invertebrate communities in temperate streams and rivers, using Before-After, Control-Impact (BACI) study designs. The study made use of routine environmental surveillance data collected as part of long-term national and international monitoring programmes at systematically-selected ROR hydroelectric power schemes and systematically-selected paired control sites. Five metrics of invertebrate community composition and six area-normalised metrics of fish community composition were analysed using linear mixed effects models. The results are discussed with respect to impacts from other sources of power, and recommendations are made for best-practice study design for future freshwater community impact studies.